Cellulose ester based quarter wave plates having normal wavelength dispersion

ABSTRACT

The present invention pertains to an optical film for use as a quarter wave plate (QWP) having a normal wavelength dispersion curve. More specifically, this invention relates to a quarter wave plate based on cellulose ester polymer and fused ring additives.

FIELD OF THE INVENTION

This invention pertains to an optical film for use as a quarter waveplate (QWP) having a normal wavelength dispersion curve. Morespecifically, this invention relates to a quarter wave plate based oncellulose ester polymer.

BACKGROUND OF THE INVENTION

It is known in the art of optical compensation that the phaseretardation of light varies according to wavelength, causing color shiftand contrast ratio reduction. This wavelength dependence (or dispersion)characteristic of the compensation film may be taken into account whendesigning an optical device so that color shift is reduced and contrastratio increased. Wavelength dispersion curves are defined as “normal (orproper)” or “reversed” with respect to the compensation film havingpositive and negative retardance (or retardation). A compensation filmwith positive retardance (positive A- or C-plate) may have a normaldispersion curve in which the value of phase retardation is increasinglypositive toward shorter wavelengths or a reversed dispersion curve inwhich the value of phase retardation is decreasingly positive towardshorter wavelengths. A compensation film with negative retardance(negative A- or C-plate) may have a normal dispersion curve in which thevalue of phase retardation is increasingly negative toward shorterwavelengths or a reversed dispersion curve in which the value of phaseretardation is decreasingly negative toward shorter wavelengths.Exemplary shapes of these curves are depicted in FIG. 1.

Wave plates are customarily named as follows in accordance with theirrefractive index profiles:

positive A-plate: n_(x)>n_(y)=n_(z); negative A-plate:n_(x)<n_(y)=n_(z); positive C-plate: n_(x)=n_(y)<n_(z); negativeC-plate: n_(x)=n_(y)>n_(z), wherein, n_(x) and n_(y) represent in-planerefractive indices, and n_(z) is the thickness refractive index.

The above wave plates are uniaxial birefringent plates. A wave plate canalso be biaxial birefringent, where n_(x), n_(y), and n_(z) all havedifferent values; it is customarily referred to as a biaxial film.

An A-plate is a wave plate commonly used as a retarder in an opticaldevice. It is a birefringent material capable of manipulating thepolarization state or phase of the light beam traveling through themedium. The A-plate optical retarder has a refractive index profile ofn_(x)>n_(y)=n_(z), wherein n_(x) and n_(y) represent in-plane refractiveindices and n_(z) represents the thickness-direction refractive index.Such a wave plate exhibits a positive in-plane retardation (R_(e)) asexpressed by R_(e)=(n_(x)−n_(y))×d, wherein d is the thickness of thewave plate. R_(e) is also often denoted as R_(o).

An A-plate having in-plane retardation (R_(e)) equal to a quarter of alight wavelength (λ), R_(e)=λ/4, is called quarter wave plate (QWP). Aquarter wave plate is capable of converting an incident linearlypolarized light into circularly polarized light. Thus, a quarter waveplate is commonly used in combination with a linear polarizer to providea circular polarizer in an optical device. Circularly polarized lighthas been used in polarized three-dimensional (3D) display systems toproduce stereoscopic image projection. Circular polarization has anadvantage over linear polarization in that viewers are able to tilttheir heads and move around naturally without seeing distorted 3Dimages. Such 3D display systems require viewers to wear glasses,commonly referred to as 3D glasses, equipped with circular polarizingfilms in order to see 3D images. Recently, there has been much increasedinterest in 3D consumer products such as TVs and computer displays.Thus, there is a demand for improved 3D glasses with circular polarizingfilms. Specifically, there is a need for a quarter wave plate havingnormal wavelength dispersion, which has been found to have the utilityfor 3D glasses to improve the viewing quality. It is known that suchquarter wave plates can be achieved by using polycarbonate or cyclicpolyolefin. However, in a device based on such quarter wave plates, acellulose ester film is required to protect the polyvinyl alcohol basedpolarizer. It would be advantageous if the quarter wave plate is basedon cellulose ester film and can also function as a protective film forthe polarizer. Accordingly, this invention is further directed toquarter wave plates based on cellulose ester.

In order to have a normal wavelength dispersion curve, the in-planeretardation (R_(e)) of a quarter wave plate should satisfy the followingequations:R _(e)(450)/R _(e)(550)>1 and R _(e)(650)/R _(e)(550)<1wherein R_(e)(450), R_(e)(550), and R_(e)(650) are in-plane retardationsat the light wavelengths of 450 nm, 550 nm, and 650 nm respectively.

The positive A-plate, however, also exhibits a negative out-of-planeretardation R_(th), which is defined as R_(th)=[n_(z)−(n_(x)+n_(y))/2]×dwith a value of [R_(e)/2] arising from its orientation. The term“|R_(e)/2|” means the absolute value of R_(e)/2. This characteristic canbe beneficial when a negative R_(th) is desirable in an optical device.For example, in a vertically aligned (VA) mode liquid crystal display(LCD), the liquid crystal molecules in the LC cell are aligned in ahomeotropic manner, which results in positive out-of-plane retardation.A wave plate with a negative R_(th), thus, can provide an out-of-planecompensation in addition to in-plane compensation in VA-LCD. In othertypes of devices, such as in-plane switch (IPS) mode LCD and 3D glasses,however, the Rth exhibited in the A-plate is not desirable since it cangive rise to phase shift in off-axis light and lead to light leakage.Thus, there exists a further need in the art to provide a quarter waveplate having reduced out-of-plane retardation for improved viewingquality.

BRIEF SUMMARY OF THE INVENTION

It has been surprisingly discovered that quarter wave plates comprisingcellulose ester polymers and additives and having normal dispersion canbe obtained.

In one aspect the present invention provides a uniaxially or biaxiallystretched optical film for use as a quarter wave plate having a normalwavelength dispersion curve comprising,

-   -   (a) a cellulose ester polymer and    -   (b) an additive having the structure below:

wherein DISK represents a disk-like moiety having a fused ringstructure, A is each independently —COO—, —OOC—, —CO—, —CONH—, —NHCO—,—O—, or —S—; Z is each independently an aryl, alkyl, ethoxylated alkyl,or ethoxylated aryl group, having 1 to 30 carbon atoms; Y is eachindependently a halogen or alkyl-, alkoxy-, or alkanoyl-group having 1to 20 carbon atoms, m=0, 1, 2, 3, or 4; n is the number of theindependent -A-Z substituents on the DISK, n=0, 1, 2, or 3; and whereinthe in-plane retardation (R_(e)) of said quarter wave plate satisfiesthe following equations:R _(e)(450)/R _(e)(550)>1 and R _(e)(650)/R _(e)(550)<1wherein R_(e)(450), R_(e)(550), and R_(e)(650) are in-plane retardationsat the light wavelengths of 450 nm, 550 nm, and 650 nm respectively.

In one aspect, the optical film in accordance with this invention is aquarter wave plate having in-plane retardation (R_(e)) of about 100-160nm at the wavelength (λ) 550 nm.

In another aspect, the optical film has an out-of-plane retardation(R_(th)) that satisfies the equation of |R_(th)|<100 nm throughout thewavelength range of about 400 nm to about 800 nm.

One aspect of the present invention provides a circular polarizercomprising

-   -   (1) a uniaxially or biaxially stretched optical film for use as        a quarter wave plate having a normal wavelength dispersion curve        comprising,        -   (a) a cellulose ester polymer and        -   (b) an additive having the structure below:

-   -    and    -   (2) a linear polarizer,        wherein DISK represents a disk-like moiety having a fused ring        structure, A is each independently —COO—, —OOC—, —CO—, —CONH—,        —NHCO—, —O—, or —S—; Z is each independently an aryl, alkyl,        ethoxylated alkyl, or ethoxylated aryl group, having 1 to 30        carbon atoms; Y is each independently a halogen or alkyl-,        alkoxy-, or alkanoyl-group having 1 to 20 carbon atoms, m=0, 1,        2, 3, or 4; n is the number of the independent -A-Z substituents        on the DISK, n=0, 1, 2, or 3; and wherein the in-plane        retardation (R_(e)) of said quarter wave plate satisfies the        following equations:        R _(e)(450)/R _(e)(550)>1 and R _(e)(650)/R _(e)(550)<1        wherein R_(e)(450), R_(e)(550), and R_(e)(650) are in-plane        retardations at the light wavelengths of 450 nm, 550 nm, and 650        nm respectively.

In one aspect of the present invention, the DISK comprises a fused ringcompound comprising two or more individual rings that are connected bysharing at least one of their sides.

In one aspect of the present invention, the fused ring of the DISKcomprises one or more of naphthalene, anthracene, phenanthrene, -pyrene,compound with structure 5, and compound with structure 6,2-naphthylbenzonate, 2,6-naphthalene dicarboxylic diester, naphthalene, abieticacid ester and mixtures thereof.

In one aspect of the present invention, the cellulose ester polymer hasan inherent viscosity of from about 0.8 to 1.9 dL/g.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph depicting the shapes of exemplary wavelengthdispersion curves for: (a) a reversed curve for positive retardation,(b) a normal curve for positive retardation, (c) a normal curve fornegative retardation and (d) a reversed curve for negative retardation.

DETAILED DESCRIPTION

In one embodiment of the present invention, there is provided auniaxially or biaxially stretched optical film for use as a quarter waveplate having a normal wavelength dispersion curve, comprising:

-   -   (a) a cellulose ester polymer and    -   (b) an additive having the structure below:

wherein DISK represents a disk-like moiety having a fused ringstructure, A is each independently —COO—, —OOC—, —CO—, —CONH—, —NHCO—,—O—, or —S—; Z is each independently an aryl, alkyl, ethoxylated alkyl,or ethoxylated aryl group, having 1 to 30 carbon atoms; Y is eachindependently a halogen or alkyl-, alkoxy-, or alkanoyl-group having 1to 20 carbon atoms, m=0, 1, 2, 3, or 4; n is the number of theindependent -A-Z substituents on the DISK, n=0, 1, 2, or 3; and whereinthe in-plane retardation (R_(e)) of said quarter wave plate satisfiesthe following equations:R _(e)(450)/R _(e)(550)>1 and R _(e)(650)/R _(e)(550)<1wherein R_(e)(450), R_(e)(550), and R_(e)(650) are in-plane retardationsat the light wavelengths of 450 nm, 550 nm, and 650 nm respectively. Theethoxylated alkyl or ethoxylated aryl comprises a moiety of−[—CH₂—CH₂—O—]_(n)—Z bonded to A, wherein n is an integer from 1 to 10.

Alternatively, Z may be independently an aryl, alkyl, ethoxylated alkyl,or ethoxylated aryl group, having 1 to 20 carbon atoms or having 1 to 15carbon atoms or having 1-10 carbon atoms or having 1 to 5 carbon atoms.

Alternatively, Y may be independently a halogen or alkyl-, alkoxy-, oralkanoyl-group having 1 to 20 carbon atoms or having 1 to 15 carbonatoms, or having 1 to 10 carbon atoms or having 1 to 5 carbon atoms.

In the ethoxylated alkyl or ethoxylated aryl comprises a moiety of−[—CH₂—CH₂—O—]_(n)—Z bonded to A, n is an integer from 1 to 10 or from 1to 8 or from 1 to 6 or from 1 to 4 or from 1 to 2. Alternatively, in theethoxylated alkyl or ethoxylated aryl comprises a moiety of−[—CH₂—CH₂—O—]_(n)—Z bonded to A, n is an integer from 2 to 10 or from 4to 10 or from 6 to 10 or from 8 to 10.

Examples of the cellulose ester polymer in (a) suitable for thisinvention include, but are not limited to, cellulose acetate (CA),cellulose acetate propionate (CAP), and cellulose acetate butyrate(CAB). The terms “cellulose ester” and “cellulose ester polymer” areused interchangeable herein. The terms refer to the condensation productfrom the reaction of a hydroxyl group on the cellulose with thecarboxylic acid group of a carboxylic acid with the formation of wateras a co-product. The cellulose esters may be randomly orregioselectively substituted. Regioselectivity can be measured bydetermining the relative degree of substitution (RDS) at C₆, C₃, and C₂in the cellulose ester by carbon 13 NMR (Macromolecules, 1991, 24,3050-3059). In conventional cellulose esters, regioselectivity isgenerally not observed and the RDS ratio of C₆/C₃, C₆/C₂, or C₃/C₂ isgenerally near 1 or less. In essence, conventional cellulose esters arerandom copolymers. In contrast, when adding one or more acylatingreagents to cellulose dissolved in an appropriate solvent, the C₆position of cellulose are acylated much faster than C₂ and C₃.Consequentially, the C₆/C₃ and C₆/C₂ ratios are significantly greaterthan 1, which is characteristic of a 6,3- or 6,2-enhancedregioselectively substituted cellulose ester.

Non-limiting examples of the DISK moiety having a fused ring structuresuch as naphthalene (structure 1), anthracene (structure 2),phenanthrene (structure 3), pyrene (structure 4), (structure 5), and(structure 6) which are shown below:

The “fused ring” structure may be understood to have two or moreindividual rings that are connected by sharing at least one of theirsides. Each individual ring in the fused ring may be substituted orunsubstituted and is preferably a six- or five-membered ring, which aretypically all-carbon. Individual rings in a fused ring may be aromaticor aliphatic. Preferred individual rings in a fused ring include, butare not limited to, aromatic rings and substituted aromatic rings,cycloaliphatic rings, substituted cycloaliphatic rings, partiallyunsaturated cycloaliphatic rings, and partially unsaturated, substitutedcycloaliphatic rings.

Non-limiting examples of the additive in (b) suitable for this inventionare 2-naphthyl benzoate, 2,6-naphthalene dicarboxylic acid ester,naphthalene, and abietic acid ester which are shown below:

wherein, R is each independently C₁-C₂₀ alkyl or C₁-C₂₀ aryl, and n iseach independently an integer ranging from 0 to 6. When n=0, R is bondeddirectly to the carboxyl group. Alternatively, R is each independentlyC₁-C₁₅ alkyl or C₁-C₁₅ aryl, and n is each independently an integerranging from 0 to 6. Alternatively, R is each independently C₁-C₁₀ alkylor C₁-C₁₀ aryl, and n is each independently an integer ranging from 0 to6. Alternatively, R is each independently C₁-C₆ alkyl or C₁-C₆ aryl, andn is each independently an integer ranging from 0 to 6.

The optical film in accordance with the present invention has a positivein-plane retardation (R_(e)) and a normal in-plane wavelength dispersioncharacteristic, in which the value of phase retardation is increasinglypositive toward shorter wavelengths. This dispersion characteristic isexpressed by the ratios of the retardations as measured at thewavelengths of 450 nm, 550 nm, and 650 nm, which satisfy the relationsof R_(e)(450)/R_(e)(550)>1 and R_(e)(650)/R_(e)(550)<1. The ratio ofR_(e)(450)/R_(e)(550) can be 1.001 to 1.1, 1.005 to 1.08, 1.01 to 1.06,or 1.02 to 1.04. The ratio of R_(e)(650)/R_(e)(550) can be 0.95 to0.999, 0.96 to 0.996, 0.97 to 0.993, or 0.98 to 0.99. The films may havea combination of the ratio of R_(e)(450)/R_(e)(550) ranging from 1.001to 1.1 with the ratio of R_(e)(650)/R_(e)(550) of 0.95 to 0.999, or 0.96to 0.996, or 0.97 to 0.993, or 0.98 to 0.99. The films may have acombination of the ratio of R_(e)(450)/R_(e)(550) ranging from 1.005 to1.08 with the ratio of R_(e)(650)/R_(e)(550) of 0.95 to 0.999, or 0.96to 0.996, or 0.97 to 0.993, or 0.98 to 0.99. The films may have acombination of the ratio of R_(e)(450)/R_(e)(550) ranging from 1.01 to1.06 with the ratio of R_(e)(650)/R_(e)(550) of 0.95 to 0.999, or 0.96to 0.996, or 0.97 to 0.993, or 0.98 to 0.99. The films may have acombination of the ratio of R_(e)(450)/R_(e)(550) ranging from 1.02 to1.04 with the ratio of R_(e)(650)/R_(e)(550) of 0.95 to 0.999, or 0.96to 0.996, or 0.97 to 0.993, or 0.98 to 0.99.

Retardation (R) of a wave plate is defined as R=Δn×d, wherein Δn is thebirefringence and d is the thickness of the wave plate. Birefringence isclassified into in-plane birefringence Δn_(e)=n_(x)−n_(y) andout-of-plane birefringence Δn_(th)=n_(z)−(n_(x)+n_(y))/2. Thus, in-planeretardation is represented by R_(e)=(n_(x)−n_(y))×d and out-of-planeretardation by R_(th)=[n_(z)−(n_(x)+n_(y))/2]×d, which is the definitionused through-out this application. It is noted that some authors use thefollowing definition R_(th)=[(n_(x)+n_(y))−n_(z)/2]×d which gives anumber of the same magnitude but with opposite sign. n_(x) is measuredin the machine direction of the film and n_(y) is measured in thetransverse direction.

Birefringence (Δn) of a wave plate may be measured by determining thebirefringence of a wave plate over a wavelength range of about 400 nm toabout 800 nm at different increments. Alternatively, birefringence maybe measured at a specific light wavelength. Throughout this description,when a birefringence or retardation relation is given without specifyinga wavelength, the relation occurs throughout the wavelength range ofabout 400 nm to about 800 nm.

Preferably, the in-plane retardation (R_(e)) of the optical compensationfilm of this invention ranges from about 80 nm to about 300 nm at thewavelength (λ) 550 nm. In a further aspect, the optical compensationfilm in accordance with this invention is a quarter wave plate havingin-plane retardation (R_(e)) of about 100-160 nm at the wavelength (λ)550 nm and having a normal in-plane dispersion characteristic. Forpurposes of this application, the term “quarter wave plate” includes Revalues ranging from about (0.7) (λ/4) to about (1.3) (λ/4), or about(0.8) (λ/4) to about (1.2) (λ/4), or from about (0.85) (λ/4) to about(1.15) (λ/4), or from about (0.9) (λ/4) to about (1.1) (λ/4).

Besides having a normal in-plane dispersion characteristic, the opticalfilm of the present invention is capable of providing a low out-of-planeretardation (R_(th)) value. The low R_(th) is desirable since it canincrease the viewing angle and improve the quality of an image. Thus,this invention further provides a wide-view optical film having anout-of-plane retardation (R_(th)) that satisfies the equation of|R_(th)|<100 nm, or <80 nm, or <50 nm, or <30 nm, or <10 nm, or <5 nmthroughout the wavelength range of about 400 nm to about 800 nm. Theterm “|R_(th)|” means the absolute value of the out-of-plane retardationvalue R_(th).

This wide-view feature due to the low R_(th) characteristic of theoptical film, when combined with the normal dispersion characteristic ofthe present invention, will provide a wide-view circular polarizer whenused in combination with a linear polarizer. Such a circular polarizercan be used in a 3D glasses device to improve the viewing quality. Thepresent invention may be used for example in 3D glasses for stereoscopicdisplay devices such as those shown in U.S. Pat. Nos. 8,228,449,8,310,528, 8,370,873, and 8,233,103.

Thus, this invention further provides a circular polarizer comprising alinear polarizer and a quarter wave plate of the present invention. Inanother embodiment, there is provided 3D glasses comprising a circularpolarizer of the present invention.

Cellulose esters can be prepared by conventional methods by contactingthe cellulose solution with one or more C1-C20 acylating reagents at acontact temperature and contact time sufficient to provide a celluloseester with the desired degree of substitution (DS) and degree ofpolymerization (DP). The cellulose esters thus prepared generallycomprise the following structure:

wherein R₂, R₃, R₆ are hydrogen, with the proviso that R₂, R₃, R₆ arenot hydrogen simultaneously, or C1-C20 straight- or branched-chain alkylor aryl groups bound to the cellulose via an ester linkage. Thus, acellulose ester can have a DS up to 3. When a cellulose ester has aDS<3, it has unreacted hydroxyl groups. The degree of un-substituted OHis customary expressed as DS_(OH).

The cellulose esters suitable for the present invention have a totaldegree of substitution of the acyl groups DS_(acyl) from about 1.5 toabout 3.0 (or DS_(OH)=0-1.5), preferably from about 2 to about 2.9 (orDS_(OH)=0.1-1.0), and more preferably from about 2.5 to about 2.8 (orDS_(OH)=0.15-0.5). Further examples of cellulose esters suitable for thepresent invention include cellulose acetate propionates having a degreeof substitution of acetate, DS_(Ac), ranging from about 0.13 to about2.34, a degree of substitution of propionate, DS_(Pr), ranging fromabout 0.85 to about 2.50, and a degree of substitution of hydroxyl,DS_(OH), ranging from about 0.32 to about 1.08. Further examples ofcellulose esters suitable for the present invention include celluloseacetate butyrate having a degree of substitution of acetate, DS_(Ac),ranging from about 0.13 to about 2.34, a degree of substitution ofbutyrate, DS_(BU), ranging from about 0.85 to about 2.50, and a degreeof substitution of hydroxyl, DS_(OH), ranging from about 0.32 to about1.08. Further examples of cellulose esters suitable for the presentinvention include cellulose acetate having a degree of substitution ofacetate, DS_(Ac) and a degree of substitution of hydroxyl, DS_(OH),ranging from about 0.16 to about 0.56.

The cellulose esters suitable for the present invention have an inherentviscosity of greater than about 0.5 dL/g, or from about 0.7 to about 1.9dL/g, or from about 0.8 to about 1.9 dig or from about 0.8 to about 1.5dL/g, or about 0.8 to about 1.2 dL/g.

The preferred acylating reagents are one or more C1-C20 straight- orbranched-chain alkyl or aryl carboxylic anhydrides, carboxylic acidhalides, diketene, or acetoacetic acid esters. Examples of carboxylicanhydrides include, but are not limited to, acetic anhydride, propionicanhydride, butyric anhydride, isobutyric anhydride, valeric anhydride,hexanoic anhydride, 2-ethylhexanoic anhydride, nonanoic anhydride,lauric anhydride, palmitic anhydride, stearic anhydride, benzoicanhydride, substituted benzoic anhydrides, phthalic anhydride, andisophthalic anhydride. Examples of carboxylic acid halides include, butare not limited to, acetyl, propionyl, butyryl, hexanoyl,2-ethylhexanoyl, lauroyl, palmitoyl, benzoyl, substituted benzoyl, andstearoyl chlorides. Examples of acetoacetic acid esters include, but arenot limited to, methyl acetoacetate, ethyl acetoacetate, propylacetoacetate, butyl acetoacetate, and tert-butyl acetoacetate. The mostpreferred acylating reagents are C2-C9 straight- or branched-chain alkylcarboxylic anhydrides selected from the group acetic anhydride,propionic anhydride, butyric anhydride, 2-ethylhexanoic anhydride,nonanoic anhydride, and stearic anhydride.

Cellulose esters useful in the present invention can be prepared by anyknown method for preparing cellulose esters. Examples of randomlysubstituted cellulose esters having various DS_(OH) are described in USapplication 2009/0096962, which is incorporated in its entirety in thisinvention.

The optical film of this invention can be made by solution casting ormelt extrusion. The solution cast film is prepared by mixing thecellulose ester polymer (a) with the additive (b) in a solvent, followedby casting of the resulting solution on a substrate. The film isobtained after the removal of the solvent. In the melt extrusion method,the cellulose ester polymer solid is mixed with the additive, followedby the extrusion of the mixture at a temperature higher than the glasstransition temperature of the polymer.

Further, to obtain certain in-plane retardation R_(e), the as-cast filmsare typically stretched. By adjusting the stretch conditions, such as,stretch temperature, stretch ratio, stretch type—uniaxial or biaxial,and controlling pre-heat time and temperature, post-stretch annealingtime and temperature, the desired R_(e), R_(th) and normal opticaldispersion can be achieved. The stretching temperature typically rangesfrom 130° C. to 200° C. The stretch ratio for MD typically ranges from1.0 to 2.0. Pre-heat time typically ranges from 10 to 300 seconds, andpre-heat temperature is typically equal to stretch temperature.Post-annealing time typically ranges from 0 to 300 seconds, andpost-annealing temperature typically ranges 10° C. to 40° C. below thestretching temperature.

In another embodiment of this invention, additives such as plasticizers,stabilizers, UV absorbers, antiblocks, slip agents, lubricants, dyes,pigments, retardation modifiers, etc. may optionally be mixed with thecellulose esters. Examples of these additives are found in USapplications 2009/0050842, 2009/0054638, and 2009/0096962.

This invention can be further illustrated by the following examples ofpreferred embodiments thereof, although it will be understood that theseexamples are included merely for purposes of illustration and are notintended to limit the scope of the invention unless otherwisespecifically indicated.

EXAMPLES Example 1 Preparation of Cellulose Ester Film 1 Having NormalDispersion Curve

A solution of cellulose ester polymer was prepared by mixing a celluloseacetate propionate (45 g) having DS of acetate (DS_(AC))=0.18, DS ofpropionate (DS_(Pr))=2.50, and DS_(OH)=0.32 in a solvent blend,methylene chloride/ethanol (90/10 wt. %), 363 g, followed by theaddition of the additive, 2-naphthyl benzoate, 4.45 g. The resultingmixture was placed on a roller for 24 hours to yield a uniform solution.

The solution prepared above was cast onto a glass plate using a doctorblade to obtain a film with the desired thickness. Casting was conductedin a fume hood by an automatic drawdown machine. The relative humidityof the hood was controlled at 40%-50%. After casting, the film wasallowed to dry for 45 minutes under a cover pan to minimize the rate ofsolvent evaporation. After the pan was removed, the film was allowed todry for 15 more minutes and subsequently peeled off from the glass. Thefree standing film thus obtained was annealed in a forced air oven at100° C. for 10 minutes and then at 120° C. for another 10 minutes. Thethickness of the resulting film was determined to be ranging from 78 to84 microns by using PosiTector®6000 (DeFelsko Corporation).

A non-constrained uniaxial stretching method was used for filmstretching. The film prepared above was mounted on a stretching machine(Karo IV laboratory film stretcher available from Brückner) equippedwith a heating chamber. The film was pre-heated for 25 seconds to reachthe stretching temperature of 150° C. and subsequently stretched in themachine direction (MD) at a speed of 7.0 mm/sec to a stretch ratioranging from 1.47 to 1.50. The transverse direction (TD) was leftun-constrained or freed.

After stretching, the retardations (R_(th) and R_(e)) of the celluloseester films (CE-1) were measured by M-2000V Ellipsometer (J. A. WoollamCo.). The results from various stretching conditions are listed in Table1, which shows the representative retardations at the wavelength 589 nm,R_(e)(589) and R_(th)(589), and the values of R_(e)(450)/R_(e)(550) andR_(e)(650)/R_(e)(550).

TABLE 1 Retardations of the Stretched Cellulose Film 1 Film ThicknessStretch After Ratio Stretching (TD × MD), at R_(e)(589), R_(th)(589),R_(e)(450)/ R_(e)(650)/ (microns) 150° C. nm nm R_(e)(550) R_(e)(550)CE-1a 80 1 × 1.48 121.8 −75.5 1.011 0.998 CE-1b 82 1 × 1.50 122.8 −73.91.012 0.997 CE-1c 84 1 × 1.50 121.4 −74.2 1.013 0.998 CE-1c 82 1 × 1.50123.2 −81.3 1.015 0.997 CE-1e 82 1 × 1.47 121.2 −73.9 1.013 0.998 CE-1f78 1 × 1.48 120.6 −80.2 1.012 0.998

Example 2 Preparation of Cellulose Ester Film 2 Having Normal DispersionCurve

A solution of cellulose ester polymer was prepared by mixing a celluloseacetate (45 g) having DS_(Ac)=2.84, DS_(OH)=0.16 in a solvent blend,methylene chloride/ethanol (92/8 wt. %), 357 g, followed by the additionof the additive, 2-naphthyl benzoate, 3.65 g. The resulting mixture wasplaced on a roller for 24 hours to yield a uniform solution.

The solution prepared above was cast onto a glass plate as described inExample 1 to obtain a film with the desired thickness. The resultingfilm (CE-2) was stretched according to Example 1 at various temperaturesto a stretch ratio ranging from 1.25 to 1.40. The results are listed inTable 2.

TABLE 2 Retardations of the Stretched Cellulose Film 2 Film ThicknessAfter Stretching Stretch Ratio R_(e)(589), R_(th)(589), R_(e)(450)/R_(e)(650)/ (microns) (TD × MD) nm nm R_(e)(550) R_(e)(550) CE-2a 74 1 ×1.30 at 165° C. 117.6 −161.1 1.014 0.999 CE-2b 78 1 × 1.25 at 125.7−162.9 1.013 0.999 167.5° C. CE-2c 80 1 × 1.40 at 170° C. 131.2 −159.71.016 0.998

Example 3 Preparation of Cellulose Ester Film 3 Having Normal DispersionCurve

A solution of cellulose ester polymer was prepared by mixing a celluloseacetate propionate (48 g) having DS_(Ac)=1.59, DS_(Pr)=0.85,DS_(OH)=0.56 in a solvent blend, methylene chloride/ethanol (90/10 wt.%), 352 g, followed by the addition of the additive, naphthalene, 4.8 g.The resulting mixture was placed on a roller for 24 hours to yield auniform solution. The solution prepared above was cast onto a glassplate as described in Example 1 to obtain a film with the desiredthickness. The resulting film (CE-3) was stretched according to Example1 at 160° C. to a stretch ratio ranging from 1.06 to 1.08. The resultsare listed in Table 3.

TABLE 3 Retardations of the Stretched Cellulose Film 3 Film ThicknessAfter Stretching Stretch Ratio R_(e)(589), R_(th)(589), R_(e)(450)/R_(e)(650)/ (microns) (TD × MD) nm nm R_(e)(550) R_(e)(550) CE-3a 82 1 ×1.08 at 160° C. 136.0 −214.7 1.008 0.995 CE-3b 80 1 × 1.07 at 160° C.117.4 −200.4 1.007 0.996 CE-3c 82 1 × 1.06 at 160° C. 122.5 −189.4 1.0100.996

Example 4 Preparation of Cellulose Ester Film 4 Having Normal DispersionCurve

A solution of cellulose ester polymer was prepared by mixing a celluloseacetate (43.2 g) having DS_(Ac)=2.44, DS_(OH)=0.56 in a solvent blend,methylene chloride/ethanol (90/10 wt. %), 352 g, followed by theaddition of the additive, 2-naphthyl benzoate, 4.8 g. The resultingmixture was placed on a roller for 24 hours to yield a uniform solution.

The solution prepared above was cast onto a glass plate as described inExample 1 to obtain a film with the desired thickness. The resultingfilm (CE-4) was stretched according to Example 1 at various temperaturesto a stretch ratio ranging from 1.06 to 1.08. The results are listed inTable 4.

TABLE 4 Retardations of the Stretched Cellulose Film 4 Film ThicknessAfter Stretching Stretch Ratio R_(e)(589), R_(th)(589), R_(e)(450)/R_(e)(650)/ (microns) (TD × MD) nm nm R_(e)(550) R_(e)(550) CE-4a 72 1 ×1.10 at 120.7 −283.6 1.033 0.984 160° C. CE-4b 80 1 × 1.05 at 119.6−305.3 1.034 0.985 165° C. CE-4c 66 1 × 1.30 at 143.7 −317.5 1.037 0.982165° C.

Example 5 Preparation of Cellulose Ester Film 5 Having Normal DispersionCurve

A solution of cellulose ester polymer was prepared by mixing a celluloseacetate propionate (43.2 g) having DS_(Ac)=0.17, DS_(Pr)=1.75,DS_(OH)=1.08 in a solvent blend, methylene chloride/ethanol (87/13 wt.%), 352 g, followed by the addition of the additive, 2-naphthylbenzoate, 4.8 g. The resulting mixture was placed on a roller for 24hours to yield a uniform solution. The solution prepared above was castonto a glass plate as described in Example 1 to obtain a film with thedesired thickness. The resulting film (CE-5) was stretched according toExample 1 at 160° C. to a stretch ratio of 1.05 and 1.10 respectively.The results are listed in Table 5.

TABLE 5 Retardations of the Stretched Cellulose Film 5 Film ThicknessAfter Stretching Stretch Ratio R_(e)(589), R_(th)(589), R_(e)(450)/R_(e)(650)/ (microns) (TD × MD) nm nm R_(e)(550) R_(e)(550) CE-5a 66 1 ×1.10 at 160° C. 174.5 −230.5 1.029 0.984 CE-5b 74 1 × 1.05 at 160° C.138.4 −231.4 1.028 0.985

Comparative Example 1 Preparation of Cellulose Ester Film 6 withoutAdditive

This example illustrates the effect of a cellulose ester film withoutthe inventive additive on the wavelength dispersion. A solution ofcellulose ester polymer was prepared according to Example 1 using thesame cellulose ester but without the addition of 2-naphthyl benzoate.Film was cast accordingly from the solution. The film (CE-6) wasstretched at 165° C. to a stretch ratio ranging from 1.50 to 1.80. Theresults are listed in Table 6. As indicated by the values ofR_(e)(450)/R_(e)(550) and R_(e)(650)/R_(e)(550), the film yielded steepreversed dispersion curves after stretching. Further, a desirable R_(e)cannot be obtained even with high stretch ratios.

TABLE 6 Retardations of the Stretched Cellulose Film 6 Film ThicknessAfter Stretching Stretch Ratio R_(e)(589), R_(th)(589), R_(e)(450)/R_(e)(650)/ (microns) (TD × MD) nm nm R_(e)(550) R_(e)(550) CE-6a 78 1 ×1.50 at 47.4 −30.1 0.757 1.128 165° C. CE-6b 74 1 × 1.60 at 42.4 −37.90.688 1.162 165° C. CE-6c 74 1 × 1.70 at 49.1 −35.9 0.657 1.178 165° C.CE-6d 72 1 × 1.80 at 55.9 −36.8 0.666 1.177 165° C. CE-6e 70 1 × 1.90 at46.2 −36.2 0.624 1.194 165° C.

Comparative Example 2 Preparation of Cellulose Ester Film 7 with aNon-Inventive Additive

This example illustrates the effect of a non-inventive additive on thewavelength dispersion. A solution of cellulose ester polymer wasprepared by mixing a cellulose acetate propionate (43.2 g) havingDS_(Ac)=0.17, DS_(Pr)=1.78, DS_(OH)=1.05 in a solvent blend, methylenechloride/ethanol (87/13 wt. %), 373 g, followed by the addition of theadditive, triacetin, 7.62 g. The resulting mixture was placed on aroller for 24 hours to yield a uniform solution.

The solution prepared above was cast onto a glass plate as described inExample 1 to obtain a film with the desired thickness. The resultingfilm (CE-7) was stretched according to Example 1 at 175° C. to a stretchratio ranging from 1.40 to 1.45. The results are listed in Table 7. Asindicated by the values of R_(e)(450)/R_(e)(550) andR_(e)(650)/R_(e)(550), the film yielded reversed dispersion curves afterstretching.

TABLE 7 Retardations of the Stretched Cellulose Film 7 Film ThicknessAfter Stretching Stretch Ratio R_(e)(589), R_(th)(589), R_(e)(450)/R_(e)(650)/ (microns) (TD × MD) nm nm R_(e)(550) R_(e)(550) CE-7a 104 1× 1.40 at 126.4 −71.2 0.986 1.005 175° C. CE-7b 102 1 × 1.45 at 137.6−77.4 0.986 1.005 175° C.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

The invention claimed is:
 1. A uniaxially or biaxially stretched opticalfilm comprising: (a) a cellulose ester polymer; and (b) an additivehaving the structure below:

wherein DISK represents a naphthalene group; A is —OOC—; Z is an arylgroup having 6 to 30 carbon atoms; Y is each independently a halogen oran alkyl, alkoxy, or alkanoyl group having 1 to 20 carbon atoms; m=0, 1,2, 3, or 4; and n=0, 1, 2, or
 3. 2. The optical film of claim 1, whereinthe cellulose ester polymer is selected from the group consisting ofcellulose acetate, cellulose acetate propionate), and cellulose acetatebutyrate.
 3. The optical film of claim 2, wherein the cellulose esterpolymer has a degree of substitution of hydroxyl (DS_(OH)) ranging from0.1 to
 1. 4. The optical film of claim 2, wherein the cellulose esterpolymer has a degree of substitution of hydroxyl (DS_(OH)) ranging from0.15 to 0.5.
 5. The optical film of claim 1, wherein the additive is2-naphthyl benzoate.
 6. The optical film of claim 1, wherein thecellulose ester polymer is selected from the group consisting ofcellulose acetate (CA), cellulose acetate propionate (CAP), andcellulose acetate butyrate (CAB) and the additive is 2-naphthylbenzoate.
 7. The optical film of claim 1, which is made by solutioncasting.
 8. The optical film of claim 1, which is made by meltextrusion.
 9. A circular polarizer comprising the optical film of claim1 and a linear polarizer.
 10. 3D glasses comprising the optical film ofclaim
 1. 11. A quarter wave plate having a normal wavelength dispersioncurve, wherein the plate comprises the optical film according to claim 1and satisfies the following equations:R _(e)(450)/R _(e)(550)>1 andR _(e)(650)/R _(e)(550)<1 wherein R_(e)(450), R_(e)(550), and R_(e)(650)are in-plane retardations at the light wavelengths of 450 nm, 550 nm,and 650 nm, respectively.
 12. The plate of claim 11, which has anin-plane retardation (R_(e)) of about 100-160 nm at the wavelength (λ)550 nm.
 13. The plate of claim 11, which has an out-of-plane retardation(R_(th)) that satisfies the equation of |R_(th)|<100 nm.
 14. The plateof claim 11, which has an in-plane retardation (R_(e)) of about 100-160nm at the wavelength (λ) 550 nm and an out-of-plane retardation (R_(th))that satisfies the equation of |R_(th)|<100 nm.
 15. The plate of claim11, wherein R_(e)(450)/R_(e)(550) is 1.001 to 1.1 andR_(e)(650)/R_(e)(550) is 0.95 to 0.999.
 16. The plate of claim 11,wherein R_(e)(450)/R_(e)(550) is 1.01 to 1.06 and R_(e)(650)/R_(e)(550)is 0.97 to 0.993.
 17. The plate of claim 11, wherein the cellulose esterpolymer is selected from the group consisting of cellulose acetate (CA),cellulose acetate propionate (CAP), and cellulose acetate butyrate(CAB); the additive is 2-naphthyl benzoate; R_(e)(450)/R_(e)(550) is1.01 to 1.06; and R_(e)(650)/R_(e)(550) is 0.97 to 0.993.
 18. A circularpolarizer comprising the plate of claim 11 and a linear polarizer. 19.3D glasses comprising the plate of claim
 11. 20. A uniaxially orbiaxially stretched optical film comprising: (a) a cellulose esterpolymer and (b) an additive having the structure below:

wherein DISK represents a disk-like moiety having a fused ringstructure, A is each independently —COO—, —OOC—, —CO—, —CONH—, —NHCO—,—O—, or —S—; Z is each independently an aryl, alkyl, ethoxylated alkyl,or ethoxylated aryl group, having 1 to 30 carbon atoms; Y is eachindependently a halogen or an alkyl, alkoxy, or alkanoyl group, having 1to 20 carbon atoms, m=0, 1, 2, 3, or 4; n is the number of theindependent -A-Z substituents on the DISK, n=0, 1, 2, or 3; and whereinthe additive is 2-naphthyl benzoate.
 21. A uniaxially or biaxiallystretched optical film comprising: (a) a cellulose ester polymer; and(b) an additive having the structure below:

wherein DISK represents a disk-like moiety having a fused ringstructure, A is each independently —COO—, —OOC—, —CO—, —CONH—, —NHCO—,—O—, or —S—; Z is each independently an aryl, alkyl, ethoxylated alkyl,or ethoxylated aryl group, having 1 to 30 carbon atoms; Y is eachindependently a halogen or an alkyl, alkoxy, or alkanoyl group, having 1to 20 carbon atoms, m=0, 1, 2, 3, or 4; n is the number of theindependent -A-Z substituents on the DISK, n=0, 1, 2, or 3; and whereinthe cellulose ester polymer is selected from the group consisting ofcellulose acetate (CA), cellulose acetate propionate (CAP), andcellulose acetate butyrate (CAB) and the additive is 2-naphthylbenzoate.
 22. The optical film of claim 20 or 21, which is made bysolution casting.
 23. The optical film of claim 20 or 21, which is madeby melt extrusion.
 24. A circular polarizer comprising the optical filmof claim 20 or 21 and a linear polarizer.
 25. 3D glasses comprising theoptical film of claim 20 or 21.